Although ceramides, which are one class of sphingolipids, are lipids that are available only in an extremely small amount with respect to the whole body, in the stratum corneum which is the outermost layer of the skin, ceramides are contained more than half of the amount of lipids, and play the important roles in the water holding function and barrier function of the skin. These ceramides are produced in the epidermal cells, then secreted to form a lamellar structure in intercellular spaces of the stratum corneum, and they maintain the stratum corneum functions. It has been extensively reported that, in skin diseases such as dry skin, rough skin, atopic dermatitis, senile xerosis and psoriasis, normal metabolism of ceramides is interfered and the amount of ceramides in the stratum corneum decreases, followed by occurring deterioration of the water holding function, barrier function and the like of the skin.
There have been attempts to develop methods of supplying decreased ceramides from an external source, but at present, the effects of the methods are not necessarily sufficient.